[U-Boot-Users] weird power-on reset problem

Jerry Van Baren gerald.vanbaren at smiths-aerospace.com
Fri Jan 17 12:55:53 CET 2003

Read your 860 manual Chapter 12.  Three times.  The difference between a 
power on reset (PORESET*) and a pushbutton (hardware reset or software 
induced reset) involves reading the configuration off the data bus and 
starting the clocks (PLL).

When power is applied, the processor reads the configuration, sets up the 
PLLs, and lets them stabilize.  On a pushbutton reset, the processor does 
NOT read the configuration and the clocks keep running however they were 
configured when power was applied.

I would be very suspicious that your PLLs are not stabilizing in 
time.  This would most likely be due to an unstable input clock or possibly 
due to unstable or noisy power supply voltages.

 From the 860 maunual, Chapter 12:

"When PORESET signal is negated, the MODCK values are sampled and internally
latched. To ensure proper operation, the PORESET signal should be asserted 
for a
minimum of 3 microseconds."

Question: Does your hardware ensure this?

"When a hard reset event occurs, the MPC860 determines its initial mode of 
operation by
sampling the values present on the data bus (D[0Ð31])..."

Question: Does your hardware ensure the data bus lines are driven properly 
until after the initial mode of operation is established?  Note that the 
manual says the processor leaves the reset state after PORESET is released 
_AND_ the PLLs are stable.  PLL instability could stretch your start up 
time, causing problems.

Question: Is your RSTCONF* being driven properly?


At 02:50 PM 1/17/2003 -0500, bren at sjtu.edu.cn wrote:
>We have a very very weird problem.
>When we power-on our MPC860 board, u-boot will stuck
>before running in RAM. But once we reset the board
>with the "reset push button", everything will be fine
>as long as we DO NOT power-off.
>Obviously, there are some differences between power-on
>reset and internal hardware reset and those differences
>are where we make some mistakes.
>Can anyone help us?
>Laudney Ren
>An expert is one who knows more and more about less and less
>until he knows absolutely everything about nothing.
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